Typical semisubmersible platforms used in offshore drilling operations are subject to pronounced heave resonance. Such resonance occurs as the platform is subjected to the natural wave action of the sea when anchored on site, and is also described in U.S. Pat. No. 4,167,147. The aforementioned patent teaches stabilization of such platforms by velocity damping of platform oscillatory displacement by applying an anti-heave force that is a function of heave velocity of the platform. The reference also cites numerous other types of apparatus and methods for controlling stability and minimizing motion of floating platforms.
According to the present invention, heave resonance can be suppressed by resonant heave dampers comprising tanks mounted in or on the columns of the platform approximately where the ambient waterline intersects the columns. Each tank has a duct leading from its bottom to a point at the bottom of the platform pontoon. The ducts may also be terminated just above or on the side of the pontoon. The tanks are in continuous communication with the water via the duct, and are in continuous communication with the atmosphere above the ambient surface of the water therein via vent holes in the top thereof. Water flowing into and out of the tanks via the ducts also has resonant characteristics. The resonant period of the damper is designed to be approximately equal to the resonant heave period of the platform.
Resonant heave dampers constructed according to the present invention are passive since natural wave action and platform motion causes water to flow into and out of the tanks via the ducts. The cross-sectional area of the ducts is substantially smaller than the cross-sectional area of the tanks, and is selectively varied at each end to increase or decrease the damping characteristics of the damper. Such dampers may also be installed on the outside of the platform column. In that configuration, they should be distinguished from the control force tanks described in U.S. Pat. No. 4,176,614, which require an air pump connected to the tank above the ambient surface of the water therein.